Tuesday, July 03, 2007

Crossroads

Global equatorial SSTs (per CPC) are generally above average with the exception of weak cool anomalies (~minus 1C) west of South America. The warmest waters with totals of ~29-31C are still present from the west central into the Northwest Pacific Ocean extending to 25 deg. N latitude. Totals across the Indian Ocean are ~29C while the tropical Atlantic Ocean has warmed to roughly 28C. The spatial horseshoe pattern of anomalous warmth continues to emanate out of the west central Pacific into the extratropics of both of hemispheres, with magnitudes in excess of 2 deg C across the Tropical Northwest Pacific (TNWP).

East of the date line along the equator, latest 5-day averaged TAO buoy data shows respectable negative anomalies at depths of approximately 100-250m, with ~minus 5C at about 150m/150W. This is an imprint from the trade wind surge of a couple of weeks ago when the oceanic thermocline was lifted. Weak warm anomalies remain just above that cool layer, and it is unclear whether or not these cool anomalies will surface. Whatever the case, for now, a tilt toward weak La-Nina conditions with respect to the interactions between the tropical oceans and circulation responses for the next 1-3 months is probable.

Tropical convective forcing has increased significantly along the equatorial west central Pacific Ocean during the last few days, with OLRA ~minus 50 W/m**2 centered ~140-150E. The MJO discussed in past writings has contributed to this increase. However, as will be discussed, I think we are losing our MJO signal. Tied to a much stronger than normal South Asian monsoon system, other intense areas of rainfall remains from the northern Arabian Sea into Southeast Asia centered ~15N. In fact, there has been a rapid increase of convection southwest of India that is probable to move north and contribute to additional flooding and other related hazards for that region. Finally, there has been a reasonably coherent convectively coupled Kelvin wave moving through the Western Hemisphere during the last week or so. I think this Kelvin wave may have contributed to the recent Andes Mountain torque of ~20 Hadleys and may also be adding to the local increase of thunderstorm activity across both the African Highlands and the Southwest Indian Ocean. Overall, one can state that the dominate region of tropical convective forcing is across the Eastern Hemisphere extending from the region of Pakistan east-southeast into the west central Pacific Ocean.

So, does the above sufficiently describe current situation of tropical convective forcing? I do not think so. I am leaning toward the notion of an emerging stationary pattern of enhanced tropical rainfall having 2 regions (as discussed in previous posts): the South Asian monsoon system and the TNWP. I think we are going to be hard pressed to see any real coherent MJOs the rest of this boreal summer. Yes, there will likely be more convectively coupled Kelvin waves bleeding into the Western Hemisphere enhancing rainfall around the Americas at times. The current flare-up of west central Pacific tropical convection is likely to shift northwest during the next 1-3 weeks, and a few tropical cyclones are probable as this occurs. There are already weak surface westerly wind anomalies along and just north of the equator in this region, with anomalous twin upper tropospheric subtropical anticyclones developing.

The global circulation response is complicated. It is only from detailed daily monitoring within the GSDM framework I think I can make some sense of it. I do want to be brief describing it. From the animations of fields such as ESRL/PSD 150mb and 250mb daily mean vector wind anomalies and totals during the last several weeks, anomalous upper tropospheric westerly wind flow of ~15-25m/s still persists across the Western Hemisphere. The largest magnitudes of these westerly anomalies are from the date line to South America, with weekly mean anomalies in excess of 25m/s. There are 2 pairs of subtropical anticylones and cyclones not only linked to the tropical forcing discussed above, but also from extratropical feedbacks. One pair of anticylones (cyclones) is ~40-60E (~80E) with the other ~140-150E (just east of the date line). Interacting with the extratropics, Rossby wave energy dispersions from the Indian Ocean (west central Pacific) pair have led to the observed strong ridges across the North Pacific (North Atlantic) during the last couple of weeks. Both of these regions of forcing, with a wet Indian Ocean and a dry west Pacific, have contributed to the intense North America ridge.

These complex interactions have allowed propagation of the well above average equatorial zonal mean upper tropospheric westerly wind flow into the subtropical and extratropical atmospheres, particularly the Southern Hemisphere given austral winter. In fact, during the last week very intense storm track activity has occurred from Australia into the South Pacific. Other ramifications has included an anomalously intense North Atlantic Jet (NAJ) with daily mean anomalies ~40m/s at times. This NAJ has contributed to the recent storminess much of the United Kingdom has experienced. Finally, the added westerly flow has also intensified the jet across southern Asia, which is now interacting with the intensifying tropical convection across the west central Pacific. It is probable that the latter will lead to a retrogression of the North American ridge possibly into the east Pacific during the next few weeks.

Including the contribution from the current easterly phase of the stratospheric Quasi-Biennial Oscillation (QBO), global relative AAM through 30 June per R1 data was slightly below normal, while just above climatology without the QBO. AAM tendency was roughly plus 20 Hadleys with a global mountain torque of ~15 Hadleys giving some contribution. With reduced trades and increased zonal mean surface westerly flow tied to intensified storm track activity across both hemispheres, global frictional torque was near average.

Summing up, I see the intensification of the west central Pacific tropical convection as a major development. This is helping to maintain some of the anomalous upper tropospheric zonal mean westerly wind flow anomalies, hence the recent positive AAM tendency. The Global Wind Oscillation (GWO) is orbiting toward GSDM Stage 2, and I would agree the current weather-climate situation is best described by that Stage. Given 10-30 day mountain-frictional torque index cycles (not discussed today) and the likelihood of additional Western Hemisphere convectively coupled Kelvin waves, a GWO circuit (magnitude unclear) through Stages 3-4 then 1 is a possibility during the next 1-3 weeks. However, I think a stationary to slowly evolving component of the global circulation will dominate until further notice. Additionally, as we go into boreal fall and the South Asian monsoon systems shut down, unlike fall 2006 when the Indian Ocean forcing was the strongest, the west central Pacific Ocean tropical convective forcing may start to dominate given the anomalous expanse and depth of the warm pool ocean water. That may start an evolution to El-Nino at some point. However, I want to emphasize that uncertainty remains huge, not only for the possibility of El-Nino development, but even the subseasonal predictions I am offering today.

There is little change from my previous 2 discussions to the international ramifications of the current weather-climate situation. Tropical cyclone risks will remain for the Bay of Bengal and even the Arabian Sea for at least the next couple of weeks. The west central and TNWP is already starting to fester with development, and could become a major concern by weeks 2-3 possibly threatening Japan ~week 3. Kelvin waves may periodically excite the East Pacific and even the Atlantic at times. In fact, there is currently an area of disturbed weather about 1500 miles east of Windward Islands per TPC that could become a depression during the next day or so. While drought persists across western Indonesia, severe thunderstorms and flooding rainfall are likely to rake areas from Pakistan into Southeast Asia and China.

The discontinuous retrogression of the North American Ridge into possibly the East Pacific Ocean (~140W) by week 3 should bring some relief from the intense heat across the Interior West. More and more ensemble prediction schemes from various global operational centers are catching on to this notion. However, much of that region particularly west of the Continental Divide looks generally dry. The Northern Plains into the Great Lakes are likely to see increasing intense to possibly severe MCS activity starting early next week. Some of this thunderstorm activity may spread southward during week 2 with trough development across roughly the Plains. For locations around Texas, slow relief from the recent flooding rainfall should occur starting next week, maybe sooner. However, for anywhere across the Plains between the Front Range of the Rockies and Mississippi River, severe flooding rainfall events could occur almost “anytime” given the persistent above average moisture transport through the Gulf of Mexico. Temperatures are most probable to tilt toward near-below normal across the middle of country with above normal summertime heat across the Deep Southeast.

Appendix

An experimental phase space plot of the GSDM (which we call a Global Wind Oscillation (GWO)) utilizing normalized relative AAM time tendency (Y-axis) and normalized relative AAM (X-axis) can be found at

These are probabilistic statements, and work is ongoing to quantify in future posts. We hope that an opportunity will arise for us to have a dedicated web page effort to expedite more objectively. The WB (2007) paper on the GSDM has been published in the February issue of MWR. I may not be able to post another discussion until late next week or early on the following week due to travel. Please check. In general, due to covering shifts and travel, my postings on this Blog will be irregular through at least August.